This invention is related to the invention disclosed in copending U.S. patent application Ser. No. 07,419,367 of MacLane et al., entitled "Method and Apparatus for Interlaced Printing", assigned to the same assignee as the present application.
The preferred apparatus for and method of practicing the present invention is associated with an ink jet printer wherein a print head scans a print medium, most typically a sheet of paper, by shuttling back and forth across the sheet of paper or by moving continuously along the sheet of paper which is held against a rotating drum. Images are formed by selectively depositing ink deposits or pixels which are located on lines and in columns. The present invention however is equally applicable to any printing process wherein a print head travels along parallel lines relative to a print medium to form a desired final image, whether that image be graphic or textual.
Early scanning print heads contain a single nozzle per color of printing. This nozzle is positioned adjacent a sheet of paper. A print head carriage then moves relative to the paper one line at a time depositing ink pixels at selected pixel locations until the entire image area has been scanned.
The length of time that it takes to make the image on the page depends on the rate of travel of the head relative to the paper and the density of pixels, and therefore lines, desired in the final image. In many commercial applications it is desirable to make the pixels small and dense enough so that the human eye during normal unaided viewing is unable to distinguish between the pixels, thereby making the image appear to be formed of continuous lines. A pixel density of 300 dots per inch (DPI) is common. This means that in order to print one inch of image across a page, 300 lines are required. Thus, for an eleven inch page, 3300 lines are required. If the print head can make ten passes over the page in one second, it would take 51/2 minutes to print the page one line at a time. This time can be reduced by printing a U.S. standard 81/2.times.11 inch sheet of paper lengthwise. This would reduce the printing time to 41/4 minutes.
One approach which has been used to reduce the printing time even further is to put multiple printing nozzles on a single printing head so that a plurality of lines are printed simultaneously. An example of a thermal printer-plotter system which has thermal resistor elements which each form pixels and which are provided in an array of adjacent elements is described in U.S. Pat. No. Re. 30,743 (originally U.S. Pat. No. 4,070,680) reissued on Sept. 15, 1981 to Shelley et al. The present invention is also usable in this type of system.
It would appear that the printing time can be reduced to any figure, at least conceptually, by making heads with any desired number of nozzles. This is conceptually possible with thermally driven jets, although the structure becomes very complex. However, it is very difficult to mount piezoelectrically driven jets on the same head so that they print in the same column on adjacent lines. A conduit connected to a reservoir of ink and controls must be connected to each nozzle. When three colors plus black are used, four jets per line are required. As the number of jets is increased, the physical requirements of head construction becomes very limiting.
In order to overcome the limitations of placing nozzles at single-line spacings, the concept of interlaced printing was developed. Representative of conventional interlaced printing are U.S. Pat. Nos. 4,069,485 issued to Martin for "Bidirectional Ink Jet Printer With Moving Record Receiver"; 4,112,469 issued to Paranjpe et al. for "Jet Drop Copying Apparatus"; 4,131,898 issued to Gamblin for "Interlacing Recorder"; 4,272,771 issued to Furukawa for "Ink Jet Printer with Multiple Nozzle Print Head and Interlacing or Dither Means"; 4,593,295 issued to Matsufuji et al.; and 4,630,076 issued to Yoshimura for "Ink-On-Demand Color Ink Jet System Printer".
FIG. 1 shows a simplified design of an exemplary printing head 20 having a plurality (five) of nozzles 22 spaced for printing simultaneously a plurality of spaced lines. Head 20 is positioned adjacent a drum 24 having a print medium, such as a sheet of paper 26, mounted on it. As the drum and sheet rotate, the head is moved at a constant velocity longitudinally along the drum so that all lines are printed. During each full revolution, the head advances the equivalent of 5 line spacings. Thus, on each subsequent revolution, the upper lines between lines printed on the prior revolution are printed in addition to new spaced lines, as shown. This head embodiment is also shown in FIG. 3 for printing by scanning back and forth across a planar print medium.
In these figures, one line spacing between the centers of adjacent lines is the inverse of the dot-per-inch density, or 1/DPI, two line spacings is 2/DPI, etc. In FIG. 2, head 20 is shown in dashed lines at the position at the edge of the sheet after completing a series of lines. The head is also shown in solid lines, in the same position on drum 24, ready to begin the next set of lines. It can be seen by the arrows that two lines of the next set of lines are interlaced with the preceding set of lines. All of the preceding lines are printed. Continued drum rotation results in all of the lines on sheet 26 being printed.
A printing system equivalent to that of FIGS. 1 and 2 is shown in FIG. 3. In this embodiment a head 30 with nozzles 32 prints alternate lines on sheet 34. However, rather than sheet 34 being on a drum and rotated, it is moved both longitudinally and laterally relative to head 30. Normally, the head shuttles back and forth across the face of sheet 34 as the sheet is advanced. After each set of lines is printed, head 30 is shifted down the sheet by an amount shown by arrows 36 and 38 which are equal in length to the width of 5 lines of print. Thus, typically head 30 is stopped at the end of each scan pass, the sheet is shifted, and the head is driven across the sheet in a reverse direction.
It can be seen that the embodiment of FIG. 3 provides in effect the same printing operation or method as that of FIGS. 1 and 2. The letters at the top of the respective head positions represent the passes in alphabetical sequence. Corresponding letters are applied to the beginning of lines printed during each pass.
In such systems, pixel data normally comes to a printer from a controller or data source in strict raster-scan format, in which the upper left-hand pixel is the first in the data stream, the next one to the right arrives next, and so on until the end of the line. The next pixel in the data stream is then the first one in the second line of the image, and the cycle repeats until the end of the image is reached. In the case of print heads which print on more than one line at a time, it is necessary to rearrange this data before it is sent to the print head so that the resultant print correctly duplicates the desired image. The resequencing scheme must be able to generate the data elements for each head nozzle at each head location.
In order for the data reformatting unit to function, as a minimum, all of the data for printing a set of pixels for a given head position must exist in a memory. Conventionally, a memory is selected which is capable of storing the entire set of data elements defining the pixels for an entire image. In this way, there is a one-to-one correspondence between pixel location on the print medium and address locations within the memory. The selection of data elements for each head position then is based simply on a mapping of the printing element positions on the memory. Such memories, however, are very large and have a correspondingly high cost. It is desirable to provide printing in a manner which will require a reduced memory size while maintaining correspondence between the memory locations and head printing elements for each head position during printing.
U.S. Pat. No. 4,069,485 issued to Martin shows an alternative system wherein lines on a document are scanned, fed into a memory, and read out to a print head for printing in the same order per line as the document was scanned. Yet another system is shown in Paranjpe et al., U.S. Pat. No. 4,112,469 in which the pixels are printed as soon as they are scanned. These systems require synchronization of scanning and printing.